create the square, select line and before putting in distentions, ctrl & right click once the sub menu opens click midpoint and hover over the line, kinda where you think it would be. hope it helps
EHM NO... Drawing a line from middlepoint means what it says. You need to draw a line from ITS middle point. so as long as you press the first point and move the mouse to left side (to say) the distance you are going to that side is simetrical the opposite side of the point to the right, when you release the button, you get the line drawn FROM MIDDLE POINT. got it? now please, make a tutorial about that.